Signs of Destroyed Dark Matter Found in Milky Way’s Core

Cosmologists say they’ve found the most compelling evidence of dark matter particles to date, deep inside the Milky Way’s core.

There, the thinking goes, the mysterious stuff is colliding to create gamma rays more frequently than anywhere else in the celestial neighborhood.

Similar studies have peppered scientific journals in recent years, but establishing the source definitively has been troublesome. That’s not the case in this study, posted Oct. 13 on the preprint server arXiv.org, says Dan Hooper, its lead author and a cosmologist at both Fermilab in Illinois and the University of Chicago.

“We’ve considered every astronomical source and nothing we know of, except dark matter, can account for the observations,” Hooper said. “No other explanation comes anywhere close.”

The claim has yet to meet the full scrutiny of other scientists, but those who have read it said they’ll be following discussions about the work closely.

“This is the first study I know of that pulls together a few threads of evidence for dark matter together with one simple particle model,” said Craig Hogan, an astrophysicist at Fermilab who wasn’t involved in the research. “It’s not proven, but it’s very exciting and deserves follow-up.”

Dark matter got its start 13.7 billion years ago in a colossal expansion of energy called the Big Bang. That energy cooled down to form normal matter, dark matter and dark energy, which now make up 4 percent, 23 percent and 73 percent of the cosmos, respectively.

Like normal matter, dark matter has gravitational pull, helping to glue billions of stars together into galaxies. But it’s called dark for a reason: The stuff hardly interacts with normal matter, making it invisible. Neutrinos are the only type of dark matter particles that have been detected in the laboratory, but they have almost zero mass and make up only a tiny fraction of dark matter’s energy-of-the-universe slice.

The huge remaining portion, astrophysicists believe, is made up of weakly interactive massive particles, or WIMPs, some 10 to 1,000 times fatter in energy than a proton. If any two particles collide, the theory goes, they destroy one another and produce gamma rays.

Hooper and his team found such high-energy death knells in more than two years of data beamed back by the Fermi space telescope, NASA’s gamma ray observatory that scans the Milky Way for high-energy action. What they found signaled the existence of colliding dark matter particles about eight to nine times heavier than protons — just outside the expected range.

“It’s lighter than many of us would have guessed, but only by a little bit,” Hooper said. “So far, we don’t have any problems with that. The range is mostly sociological and not written in stone.”

The team found the signals in data from a 100-light-year-wide zone of the core. They looked there, Hooper explained, because it’s one of dark matter’s favorite hangout spots and, in the Milky Way’s case, the stuff is 100,000 times more concentrated than at the galaxy’s outskirts. In short, the galaxy’s core is a demolition derby for dark matter.

As tantalizing as the evidence may be, however, other scientists want to see Carl Sagan’s bill of “extraordinary claims require extraordinary evidence” fulfilled before they climb on board. In simple terms, that means definitive evidence both in nature and in the laboratory.

“No one has produced Sagan-class evidence,” said Michael Turner, a cosmologist at the University of Chicago who wasn’t involved with the study. “The hardest part to accepting this is that you have to exclude astrophysical explanations, and nature is very, very clever. It could be something we just haven’t thought of yet.”

The good news, says Turner, is that several promising dark-matter detection experiments are underway. In particular, deep-underground detectors like CoGeNT, which may have seen signs of WIMPs in recent years, could lend Hooper a hand.

“This decade is the decade of dark matter. The problem is ripe to solve,” Turner said. “We’ve gotten to this point where all of these detectors are looking in the right places.”

Hooper agrees on both counts, but says no astrophysicist he spoke with could explain the phenomenon. As for verifying dark matter’s existence in the laboratory, he suspects it’s only a matter of weeks before his findings are backed up or trounced.

“I haven’t been this excited about being a cosmologist ever before,” Hooper said.

Image: A gamma-ray map of the Milky Way recorded over the course of 3 months. NASA/DOE/Fermi LAT Collaboration (high-res version)

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